Positive working non-silver systems in which an originally coloured species is decolourised (bleached) in an imagewise manner upon exposure to light have received a considerable amount of attention. A large variety of dyes and activators have been disclosed for such systems, see, for example, J. Kosar, Light Sensitive Systems, page 387, Wiley, New York 1965.
The reaction relies on the fact that the dye absorption is sensitising the dye's own destruction or decolourisation, for example a yellow dyes absorbs blue light; the excited dye thus formed reacts with an activator which releases the species to bleach the dye. Similarly green light would destroy the magenta and red light the cyan dyes.
This dye bleach-out process is thus capable of producing colour images in a simple way. However, in spite of its apparent simplicity, the bleach-out process poses a number of problems. In particular, the purity of the whites in the final image leaves much to be desired, image stability may not be good and a fixing step may be required to stabilise the image.
One imaging system discloses a radiation-sensitive element capable of recording an image upon image-wise exposure to radiation of selected wavelength, the element comprising, as the image-forming components, an effective amount of a bleachable dye in reactive association with an iodonium ion.
The element is capable of recording a positive image simply upon exposure to radiation of selected wavelength; the radiation absorbed by the dye which is in reactive association with an iodonium ion causes the dye to bleach. The dyes are believed to sensitise spectrally the reduction of the iodonium ion through the radiation absorbed by the dyes associated with the iodonium ion. Thereafter the element may be stabilised to fix the image by destruction of the iodonium ion or by separation of the dye relative to the iodonium ion.
The dyes used may be of any colour and any chemical class which is capable of bleaching upon exposure to radiation of selected wavelength in the presence of an iodonium ion.
By a suitable selection of dye an element may be prepared which is sensitive to radiation of a selected wavelength band within the general range 300 to 1100 nm, the particular wavelength and the width of the band depending upon the absorption characteristics of the dye. In general, where a dye has more than one absorption peak it is the wavelength corresponding to the longest wavelength peak at which one would choose to irradiate the element.
Elements intended for the production of images from radiation in the visible region (400 to 700 nm) will contain dyes which will bleach from a coloured to a substantially colourless or very pale state. In practice, such bleachable dyes will undergo a change such that the transmission optical density at the .lambda..sub.max will drop from 1.0 or more to less than 0.09, preferably less than 0.05. The dyes will generally be coated on the support to provide an optical density of about 3.0 or more.
In the case of elements sensitive to ultraviolet radiation (300 to 400 nm) the dyes will not normally be coloured to the eye and there may be no visible change upon exposure to ultraviolet radiation and bleaching. The image-wise exposed elements may be used as masks for further ultraviolet exposure after fixing.
Infrared sensitive elements contain dyes having an absorption peak in the wavelength range 700 to 1100 nm. These dyes may also have absorption peaks in the visible region before and/or after bleaching. Thus, as well as providing a means for obtaining masks for subsequent infrared exposure in a similar manner to the ultraviolet masks, infrared sensitive elements may record a visible image upon image-wise exposure to infrared radiation.
Exposure may be achieved with a wide variety of sources including incandescent, gas discharge and laser sources. For laser scanning applications the laser beam may need to be focussed in order to achieve sufficient exposure.
The dyes used may be anionic, cationic or neutral. Anionic dyes give very good photosensitisation which is believed to be due to an intimate reactive association between the negatively charged dye and the positively charged iodonium ion. Also anionic dyes may readily be mordanted to cationic polymer binders and it is relatively simple to remove surplus iodonium ions in an aqueous bath in a fixing step if the mordanting polymer is cationic. However, neutral dyes also give good results and are preferred over cationic dyes for overall photosensitivity. Cationic dyes are least preferred since it is more difficult to achieve intimate reactive association between the positively charged dye and iodonium ion, and selective removal of iodonium ion after imaging is more difficult.
The bleachable dyes may be generically referred to as polymethine dyes which term characterises dyes having at least one electron donor and one electron acceptor group linked by methine groups or aza analogues. The dyes have an oxidation potential between 0 and +1 volt, preferably between +0.2 and +0.8 volt. The bleachable dyes may be selected from a wide range of known classes of dyes including allopolar cyanine dye bases, complex cyanine, hemicyanine, merocyanine, azine, oxonol, streptocyanine and styryl.
The dye and iodonium system has its greatest sensitivity at the .lambda..sub.max of the longest wavelength absorbance peak. Generally, it is necessary to irradiate the system with radiation of wavelength in the vicinity of this .lambda..sub.max for bleaching to occur. Thus, a combination of coloured dyes may be used, e.g. yellow, magenta and cyan, in the same or different layers in an element and these can be selectively bleached by appropriate visible radiation to form a full colour image. Monochromatic or polychromatic images may be produced using the photosensitive materials with relatively short exposure times in daylight or sunlight or even artificial sources of light (e.g. fluorescent lamps or laser beams). The exposure time, for adequate results, for example when using an 0.5 kW tungsten lamp at a distance of 0.7 m, may be between 1 second to 10 minutes.
The iodonium salts used in the imaging system are compounds consisting of a cation wherein a positively charged iodine atom bears two covalently bonded carbon atoms, and any anion. Preferably the acid from which the anion is derived has a pKa &lt;5. The preferred compounds are diaryl, aryl/heteroaryl or diheteroaryl iodonium salts in which the carbon-to-iodine bonds are from aryl or heteroaryl groups. Aliphatic iodonium salts are not normally thermally stable at temperatures above 0.degree. C. However, stabilised alkyl phenyl iodonium salts such as those disclosed in Chem. Lett. 1982, 65-6 are stable at ambient temperatures and may be used.
The bleachable dye and iodonium salt are in reactive association on the support. Reactive association is defined as such physical proximity between the compounds as to enable a chemical reaction to take place between them upon exposure to light. In practice, the dye and iodonium salt are in the same layer or in adjacent layers on the support.
In general, the weight ratio of bleachable dye to iodonium salt in the element is in the range from 1:1 to 1:50, preferably in the range from 1:2 to 1:10.
The bleachable dye and iodonium salt may be applied to the support in a binder. Suitable binders are transparent or translucent, are generally colourless and include natural polymers, synthetic resins, polymers and copolymers, and other film forming media. The binders may range from thermoplastic to highly cross-linked, and may be coated from aqueous or organic solvents or emulsions.
Suitable supports include transparent film, e.g. polyester, paper e.g. baryta-coated photographic paper, and metallised film. Opaque vesicular polyester films are also useful.
The fixing of the radiation-sensitive elements may be effected by destruction of the iodonium ion by disrupting at least one of the carbon-to-iodine bonds since the resulting monoaryl iodine compound will not react with the dye. The conversion of the iodonium salt to its non-radiation sensitive form can be effected in a variety of fashions. Introduction of ammonia and amines in reactive association with the iodonium ion, or a reaction caused on heating, or UV irradiation of a nucleophilic anion such as I.sup..crclbar., Br.sup..crclbar., Cl.sup..crclbar., BAr.sub.4.sup..crclbar. (tetra-arylboronide), ArO.sup..crclbar. (e.g. phenoxide), or 4NO.sub.2 C.sub.6 H.sub.4 CO.sub.2.sup..crclbar., with the iodonium ion, will effect the conversion.
An alternative method of achieving post-imaging stabilisation or fixing is to remove the iodonium ion from reactive association with the dye by washing with an appropriate solvent. For example, in the case of elements using mordanted oxonol dyes and water soluble iodonium salts formulated in gelatin, after imaging, the iodonium salt is simply removed by an aqueous wash, which leaves the immobilised dye in the binder. The dye stability to light is then equivalent to that of the dye alone. An element in which the dye and iodonium salt is formulated in polyvinylpyridine may be treated with aliphatic ketones to remove the iodonium salt and leave the dye in the binder.
The elements may be used as transparencies for use with overhead projectors, for making enlarged or duplicate copies of colour slides and for related graphics or printing applications, such as pre-press colour proofing materials.
Dye diffusion transfer systems are known and are becoming increasingly important in colour photography (see C. C. Van de Sande in Angew Chem. 1983, 22, 191-209). These systems allow "rapid access" colour images without a complicated processing sequence. The construction of these colour materials may be donor-receptor type (e.g. Ektaflex commercially available from Kodak) integral peel-apart type (e.g. Polaroid, E. H. Land, H. G. Rogers, V. K. Walworth in J. Sturge Nebelette's Handbook of Photography and Reprography, 7th Ed. 1977, Chapter 12), or integral single sheet type (e.g. Photog. Sci. and Eng., 1976, 20, 155). Silver halide diffusion transfer systems are also known (e.g. E. H. Land. Photog. Sci. and Eng., 1977, 21, 225). Examples of diffusion transfer fixing in non-silver, dye-forming reactions employing solvent application to effect the transfer are disclosed in U.S. Pat. No. 3,598,583. This patent also describes a full-colour imaging element, applicable for preparation of colour proofs, fixed by transfer of dye precursors in register to a receptor. Other examples of non-silver diffusion transfer imaging systems are disclosed in British Patent Specification Nos. 1 057 703, 1 355 618 and 1,378,898. The latter two Patents also disclose transfer of dye images under the influence of dry heat.
It has now been found that certain dyes which are bleachable upon exposure to radiation in the presence of iodonium ion are susceptible to diffusion or sublimation transfer and this property may be utilised to separate such dyes from the iodonium ion and produce a clean, stable image by transfer from a radiation-sensitive layer to a receptor layer or separate receptor element.